Methods and Systems for LC-MS/MS Proteomic Genotyping

ABSTRACT

Disclosed are methods and systems using liquid chromatography/tandem mass spectrometry (LC-MS/MS and 2D-LC-MS/MS) for the proteomic analysis of genotypes. In certain embodiments, samples used in the analysis comprise dried bodily fluids.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/428,532, filed May 31, 2019, which claims the benefit of and priorityto U.S. Provisional Patent Application No. 62/679,133, filed Jun. 1,2018, U.S. Provisional Patent Application No. 62/679,286, filed Jun. 1,2018, and U.S. Provisional Patent Application No. 62/680,256, filed Jun.4, 2018. These applications are each incorporated herein by reference intheir entireties.

FIELD OF INVENTION

The presently disclosed subject matter relates to methods and systemsfor LC-MS/MS proteomic genotyping.

BACKGROUND

There is often a need in the medical field to determine the genotype ata particular genetic locus. For example, two genetic variants of Apolipoprotein L1 (ApoL1), termed G1 and G2, are present in a largefraction of the African American population. Individuals possessing twoApoL1 risk variants (G1/G1, G2/G2, or G1/G2) are at an increased risk ofdeveloping non-diabetic kidney disease. Further, it has beendemonstrated that kidney transplant recipients experience earlierallograft failure on average when using donor organs from AfricanAmericans with two ApoL1 risk variants. Protein sequencing by proteomicmethods may identify genotypes by identifying the corresponding proteinvariant coded by the gene of interest, which can have practical benefitsrelative to conventional gene (DNA) sequencing and transcript (RNA)sequencing. This may be accomplished by proteomic analysis of the bodyfluid containing the protein variant; however, production of dryspecimens from bodily fluids for proteomic analysis may be preferredwhen remote or self-sample collection is advantageous.

SUMMARY

In some embodiments, the presently disclosed subject matter providesmethods and systems for the proteomic determination of a genotype ofinterest in a sample. The method may be embodied in a variety of ways.

For example, disclosed is a method for determining the genotype of agene of interest in a subject, the method comprising: providing abiological sample from the subject; and using mass spectrometry todetect at least one allele specific surrogate peptide present in thesample; and determining the genotype based on the at least one allelespecific surrogate peptide (i.e., the proteomic profile). In anembodiment, the method may comprise providing a body fluid from asubject, wherein the bodily fluid contains a protein derived from thegene of interest; depositing the body fluid on a solid substrate,wherein the fluid is allowed to dry to produce a dry specimen; digestingthe dry specimen to generate at least one allele specific surrogatepeptide; determining the genotype of the body fluid based on thepresence or absence of the at least one allele specific surrogatepeptide (i.e., the proteomic profile) by mass spectrometry. In anembodiment, the mass spectrometry comprises liquid chromatography tandemmass spectrometry (LC-MS/MS). In some embodiments, the liquidchromatography comprises high performance liquid chromatography (HPLC).

The method may include the step of digesting a protein or peptidesderived from the gene of interest in the biological sample to generateallele specific surrogate peptides specific to the protein variant codedby the allele (i.e., genotype), wherein the surrogate peptides comprisea proteomic profile. In certain embodiments, the method may furthercomprise detecting the presence of at least one common surrogate peptidethat is common to all protein variants coded by the alleles of the geneof interest. Thus, in certain embodiments, the protease digestion mayproduce common surrogate peptides (or qualifying peptides) that arecommon to the various alleles present for the gene. In an embodiment,the presence or absence of the at least one allele-specific surrogatepeptide is determined by comparing a measured response for at least oneallele-specific surrogate peptide to a measured response for at leastone common surrogate peptide.

In some embodiments, the digestion step may be performed using theprotease trypsin. Or, other proteases or chemicals for proteinhydrolysis can be used. Also, in some embodiments, the protein derivedfrom the gene of interest is denatured prior to digestion to facilitatedigestion.

In some embodiments, an internal standard(s) may be used for thedetected peptide or peptides. For example, the presence or absence ofthe allele specific surrogate peptides specific to a genotype may bedetermined by comparing the measured responses of the surrogate peptideto the responses of its internal standard added to the individual'ssample. In some embodiments, the internal standard is a stableisotope-labeled analogue of the allele specific surrogate peptide.Additionally and/or alternatively, the presence or absence of the atleast one common surrogate peptide may be determined by comparing themeasured responses of the surrogate peptide to the responses of itsstable isotope-labeled analogue that is added as an internal standard tothe individual's sample.

In an embodiment, the measured response is the peak area ratio for aMS/MS transition characteristic of at least one fragment ion generatedfrom the allele specific surrogate peptide. Additionally and/oralternatively, the measured response may be the peak area ratio for aMS/MS transition characteristic of at least one fragment ion generatedfrom the common surrogate peptide. For example, in an embodiment,fragment ions are produced for each of the peptides during MS/MS. Wherethe fragment ion is from the C-terminus, the fragment ion may be denoted“y”. Where the fragment ion is from the N-terminus, the fragment ion maybe denoted “b”. Additionally, the fragment ion may be identified by thenumber of amino acid residues. For example, for a peptide having thesequence LNILNNNYK (SEQ ID NO. 4), the fragment ion y4 would have thesequence NNYK (SEQ ID NO. 13). In an embodiment, the results may bereported as the peak area ratio (PAR) for the light (unlabeled) peptide(e.g., after tryp sin digestion of the protein) to the PAR for the heavy(stable isotope labeled) peptide. This can provide a normalized responsefor the unlabeled tryptic peptide.

The method may be applied to any protein. In an embodiment, the proteinis ApoL1. For example, for ApoL1, the allele specific surrogate peptidemay have the amino acid sequence LNILNNNYK (SEQ ID NO. 4) derived fromthe wild-type allele (SEQ ID NO. 1), or may have the amino acid sequenceLNMLNNNYK (SEQ ID NO. 5) derived from the G1 allele (SEQ ID NO. 2), ormay have the amino acid sequence LNILNNK (SEQ ID NO. 6) derived from theG2 allele (SEQ ID NO. 3). Also for ApoL1, the common surrogate peptidemay have the amino acid sequence of SETAEELK (SEQ ID NO. 7) and/orVAQELEEK (SEQ ID NO. 8) wherein the common surrogate peptide is presentin each of the wild-type, G1 or G2 alleles. For the measurement of thesepeptides, the mass spectrometry may measure at least one of thetransitions in Table 3. Also, for ApoL1, the presence or absence of theat least one allele specific surrogate peptide may be determined bycomparing a measured response for the at least one allele specificsurrogate peptide to a measured response for a stable isotope-labeledanalogue listed in Table 2 of the at least one allele specific surrogatepeptide.

As disclosed herein, a variety of body fluids from a subject may containa protein variant coded by the gene allele of interest. In some cases,the body fluid is dried plasma or dried red blood cells, which areproduced by addition of blood to a substrate that immobilizes red bloodallowing for separation of the plasma. In some embodiments, thebiological sample is dried blood, dried urine, or dried saliva collectedon a suitable substrate.

Also disclosed are systems for performing the proteomic methodsdisclosed herein. For example, disclosed is a system for determining thegenotype of a gene of interest in a subject, the system comprising: adevice for providing a dried body fluid comprising a protein derivedfrom the gene of interest; a station for subjecting the dried body fluidto digestion to generate at least one allele specific surrogate peptideand optionally, at least one common surrogate peptide for the protein;optionally, a station for chromatographic purification of the at leastone allele specific surrogate peptide and the optional at least onestable isotope-labeled analogue of the at least one common surrogatepeptide; and a station for analyzing the at least one allele specificsurrogate peptide by mass spectrometry to determine the presence oramount of the at least one allele specific surrogate peptide in thebiological sample.

In an embodiment, the device for providing a dried bodily fluidcomprises a device to immobilize and separate red blood cells fromplasma on a substrate. Also, in an embodiment, the system may furthercomprise a station for adding a stable isotope labeled internal standardfor the at least one allele specific surrogate peptide and optionally,the at least one common surrogate peptide for the protein. Also, in somecases the station for mass spectrometry comprises a tandem massspectrometer and/or the station for chromatography comprises highperformance liquid chromatography (HPLC). The system may behigh-throughput in nature. Also, in some cases, at least one of thestations is controlled by a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the non-limiting accompanying drawings, which are notnecessarily drawn to scale.

FIG. 1 shows the amino acid sequence of ApoL1 wild-type (SEQ ID NO. 1),G1 (SEQ ID NO. 2), and G2 (SEQ ID NO. 3) protein variants, as well asthe allele specific surrogate peptides produced by trypsin digestion ofeach protein variant in accordance with an embodiment of the disclosure.The wild-type (WT) specific surrogate peptide is LNILNNNYK (SEQ ID NO.4). The G1 specific surrogate peptide is LNMLNNNYK (SEQ ID NO. 5). TheG2 specific surrogate peptide is LNILNNK (SEQ ID NO. 6).

FIG. 2 shows certain ApoL1 common surrogate peptides produced by trypsindigestion of the WT, G1 and G2 variants in accordance with an embodimentof the disclosure. Qualifying peptide 1 is SETAEELK (SEQ ID NO. 7).Qualifying peptide 2 is VAQELEEK (SEQ ID NO. 8).

FIG. 3 shows certain methods for separation and drying of red bloodcells and plasma from deposition of whole blood onto a laminar flowpaper substrate to produce dried plasma, as well as time course forcollection and drying of whole blood (i.e. dried blood spots) inaccordance with an embodiment of the disclosure.

FIG. 4 shows a method for the determination of ApoL1 genotype inaccordance with an embodiment of the disclosure.

FIG. 5 shows a system for high-throughput proteomic genotyping inaccordance with an embodiment of the disclosure.

FIG. 6 shows surrogate peptide detection patterns for ApoL1 alleles inaccordance with an embodiment of the disclosure, where circles indicatepositives and diamonds or empty boxes indicate negatives.

FIG. 7 shows a qualitative assignment of ApoL1 genotypes in accordancewith an embodiment of the disclosure, where circles indicate positivesand diamonds or empty boxes indicate negatives. In the figure, q1 and q2represent the qualifying peptides

FIG. 8 shows a semi-quantitative assessment of ApoL1 genotypes inaccordance with an embodiment of the disclosure.

FIG. 9 shows a comparison of proteomic genotyping using dried plasma andliquid plasma as compared to Sanger DNA sequencing of blood.

FIG. 10 shows an assessment of the ApoL1 wild-type (WT) peptide as afunction of increasing hemoglobin concentration in accordance with anembodiment of the disclosure. Assessments were performed for sampleshaving 5 mg/dL hemoglobin, 518 mg/dL hemoglobin, 2073 mg/dL hemoglobinand 8300 mg/dL hemoglobin as shown above the plots for both thewild-type (WT) peptide, and a heavy stable isotope internal standard(IS).

FIG. 11 shows a titration of ApoL1 WT, G1 and G2 peptides as a functionof dried plasma immobilized on a laminar flow paper and harvested from ¼inch punches in accordance with an embodiment of the disclosure. Thex-axis indicates whether the sample was liquid plasma (20 uL), or driedplasma isolated using a solid substrate, where the approximate size andshape of the sampled substrate (i.e., “punches”) is shown.

FIG. 12 shows a trypsin digestion time course to generate the qualifyingpeptides and the allele specific peptides using either liquid plasma(black circles) or dried plasma (gray circles) in accordance with anembodiment of the disclosure. The amount of digestion at the 30 min timepoint is indicated with the vertical shading.

FIG. 13 shows stability of the wild-type and allele specific surrogatepeptides in dry plasma stored at either 23° C. or 37° C. for up to 28days in accordance with an embodiment of the disclosure.

FIG. 14 shows a system for proteomic genotyping by LC-MS/MS inaccordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

The presently disclosed subject matter now will be described more fullyhereinafter with reference to the accompanying description and drawings,in which some, but not all embodiments of the presently disclosedsubject matter are shown. The presently disclosed subject matter can beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Many modifications and other embodiments of the presently disclosedsubject matter set forth herein will come to mind to one skilled in theart to which the presently disclosed subject matter pertains having thebenefit of the teachings presented in the foregoing descriptions and theassociated drawings. Therefore, it is to be understood that thepresently disclosed subject matter is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation. Thedisclosure utilizes the abbreviations shown below.

Abbreviations

-   -   APCI=atmospheric pressure chemical ionization    -   CV=Coefficient of variance    -   EDTA=Ethylenediaminotetraacetic acid    -   HTLC=high turbulence (throughput) liquid chromatography    -   HPLC=high performance liquid chromatography    -   IS=internal standard    -   LC=liquid chromatography    -   LLE=liquid-liquid extraction    -   LOB=limit of blank    -   LOQ=limits of quantification    -   LLOQ=lower limit of quantification    -   MS/MS=tandem mass spectrometry    -   N=number of replicates    -   N/A=not applicable    -   PAR=peak area ratio    -   QC=quality control    -   R=correlation coefficient

Abbreviations

-   -   SST=system suitability test    -   ULOQ=upper limit of quantification    -   2D-LC-MS/MS=two-dimensional liquid chromatography hyphenated to        tandem mass spectrometry    -   (LC)-LC-MS/MS=two-dimensional liquid chromatography tandem        hyphenated to mass spectrometry    -   (LC)-MS/MS=liquid chromatography hyphenated to tandem mass        spectrometry

Definitions

While the following terms are believed to be well understood by one ofordinary skill in the art, the following definitions are set forth tofacilitate explanation of the presently disclosed subject matter. Otherdefinitions are found throughout the specification. Unless otherwisedefined, all technical and scientific terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich this presently described subject matter belongs.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all subranges subsumedtherein. For example, a stated range of “1 to 10” should be consideredto include any and all subranges between (and inclusive of) the minimumvalue of 1 and the maximum value of 10; that is, all subranges beginningwith a minimum value of 1 or more, e.g. 1 to 6.1, and ending with amaximum value of 10 or less, e.g., 5.5 to 10. Additionally, anyreference referred to as being “incorporated herein” is to be understoodas being incorporated in its entirety.

The terms “a”, “an”, and “the” refer to “one or more” when used in thisapplication, including the claims. Thus, for example, reference to “acell” includes a plurality of such cells, unless the context clearly isto the contrary (e.g., a plurality of cells), and so forth.

As used herein, the term “biomarker” or a “biomarker of interest” is anybiomolecule that may provide biological information about thephysiological state of an organism. In certain embodiments, the presenceor absence of the biomarker may be informative. In other embodiments,the level of the biomarker may be informative. In an embodiment, thebiomarker of interest may comprise a peptide, a hormone, a nucleic acid,a lipid or a protein. Or, other biomarkers may be measured. In anembodiment, the biomarker may comprise a peptide derived from ApoL1. Invarious embodiments the peptide may be a wild-type peptide, a G1variant, or a G2 variant.

As used herein, the term “body fluid” refers to a liquid sample obtainedfrom a biological source, including, but not limited to, an animal, acell culture, an organ culture, and the like. Suitable samples includeblood, plasma, serum, urine, saliva, tear, cerebrospinal fluid, or otherliquid aspirate, all which are capable deposition onto a substrate forcollection and drying. In an embodiment, the body fluid may be separatedon the substrate prior to drying. For example, blood may be depositedonto a paper substrate and/or laminar flow device which limits migrationof red blood cells allowing for separation of the blood plasma fractionprior to drying in order to produce a dried plasma sample for analysis.

As used herein, the terms “individual” and “subject” are usedinterchangeably. A subject may comprise an animal. Thus, in someembodiments, the biological sample is obtained from a mammalian animal,including, but not limited to a dog, a cat, a horse, a rat, a monkey,and the like. In some embodiments, the biological sample is obtainedfrom a human subject. In some embodiments, the subject is a patient,that is, a living person presenting themselves in a clinical setting fordiagnosis, prognosis, or treatment of a disease or condition.

As used herein, a subject may comprise an animal. Thus, in someembodiments, the biological sample is obtained from a mammalian animal,including, but not limited to a dog, a cat, a horse, a rat, a monkey,and the like. In some embodiments, the biological sample is obtainedfrom a human subject. In some embodiments, the subject is a patient,that is, a living person presenting themselves in a clinical setting fordiagnosis, prognosis, or treatment of a disease or condition.

As used herein, the terms “purify” or “separate” or derivations thereofdo not necessarily refer to the removal of all materials other than theanalyte(s) of interest from a sample matrix. Instead, in someembodiments, the terms “purify” or “separate” refer to a procedure thatenriches the amount of one or more analytes of interest relative to oneor more other components present in the sample matrix. In someembodiments, a “purification” or “separation” procedure can be used toremove one or more components of a sample that could interfere with thedetection of the biomarker of interest, for example, one or morecomponents that could interfere with detection of an analyte by massspectrometry.

As used herein, “chromatography” refers to a process in which a chemicalmixture carried by a liquid or gas is separated into components as aresult of differential distribution of the chemical entities as theyflow around or over a stationary liquid or solid phase.

As used herein, “liquid chromatography” (LC) means a process ofselective retardation of one or more components of a fluid solution asthe fluid uniformly percolates through a column of a finely dividedsubstance, or through capillary passageways. The retardation resultsfrom the distribution of the components of the mixture between one ormore stationary phases and the bulk fluid, (i.e., mobile phase), as thisfluid moves relative to the stationary phase(s). “Liquid chromatography”includes reverse phase liquid chromatography (RPLC), high performanceliquid chromatography (HPLC) and high turbulence liquid chromatography(HTLC).

As used herein, the term “HPLC” or “high performance liquidchromatography” refers to liquid chromatography in which the degree ofseparation is increased by forcing the mobile phase under pressurethrough a stationary phase, typically a densely packed column.

The chromatographic column typically includes a medium (i.e., a packingmaterial) to facilitate separation of chemical moieties (i.e.,fractionation). The medium may include minute particles. The particlesmay include a bonded surface that interacts with the various chemicalmoieties to facilitate separation of the chemical moieties such as thebiomarker analytes quantified in the experiments herein. One suitablebonded surface is a hydrophobic bonded surface such as an alkyl bondedsurface. Alkyl bonded surfaces may include C-4, C-8, or C-18 bondedalkyl groups, preferably C-18 bonded groups. The chromatographic columnmay include an inlet port for receiving a sample and an outlet port fordischarging an effluent that includes the fractionated sample. In themethod, the sample (or pre-purified sample) may be applied to the columnat the inlet port, eluted with a solvent or solvent mixture, anddischarged at the outlet port. Different solvent modes may be selectedfor eluting different analytes of interest. For example, liquidchromatography may be performed using a gradient mode, an isocraticmode, or a polytyptic (i.e. mixed) mode. In one embodiment, HPLC mayperformed on a multiplexed analytical HPLC system with a C18 solid phaseusing isocratic separation with water: methanol as the mobile phase.

As used herein, the term “analytical column” refers to a chromatographycolumn having sufficient chromatographic plates to effect a separationof the components of a test sample matrix. Preferably, the componentseluted from the analytical column are separated in such a way to allowthe presence or amount of an analyte(s) of interest to be determined. Insome embodiments, the analytical column comprises particles having anaverage diameter of about 5 μm. In some embodiments, the analyticalcolumn is a functionalized silica or polymer-silica hybrid, or apolymeric particle or monolithic silica stationary phase, such as aphenyl-hexyl functionalized analytical column.

Analytical columns can be distinguished from “extraction columns,” whichtypically are used to separate or extract retained materials fromnon-retained materials to obtained a “purified” sample for furtherpurification or analysis. In some embodiments, the extraction column isa functionalized silica or polymer-silica hybrid or polymeric particleor monolithic silica stationary phase, such as a Poroshell SBC-18column.

The term “heart-cutting” refers to the selection of a region of interestin a chromatogram and subjecting the analytes eluting within that regionof interest to a second separation, e.g., a separation in a seconddimension.

The term “matrix-assisted laser desorption ionization,” or “MALDI” asused herein refers to methods in which a non-volatile sample is exposedto laser irradiation, which desorbs and ionizes analytes in the sampleby various ionization pathways, including photo-ionization, protonation,deprotonation, and cluster decay. For MALDI, the sample is mixed with anenergy-absorbing matrix, which facilitates desorption of analytemolecules.

The term “surface enhanced laser desorption ionization,” or “SELDI” asused herein refers to another method in which a non-volatile sample isexposed to laser irradiation, which desorbs and ionizes analytes in thesample by various ionization pathways, including photo-ionization,protonation, deprotonation, and cluster decay. For SELDI, the sample istypically bound to a surface that preferentially retains one or moreanalytes of interest. As in MALDI, this process may also employ anenergy-absorbing material to facilitate ionization.

The term “electrospray ionization,” or “ESI,” as used herein refers tomethods in which a solution is passed along a short length of capillarytube, to the end of which is applied a high positive or negativeelectric potential. Upon reaching the end of the tube, the solution maybe vaporized (nebulized) into a jet or spray of very small droplets ofsolution in solvent vapor. This mist of droplet can flow through anevaporation chamber which is heated slightly to prevent condensation andto evaporate solvent. As the droplets get smaller the electrical surfacecharge density increases until such time that the natural repulsionbetween like charges causes ions as well as neutral molecules to bereleased.

The term “ionization” and “ionizing” as used herein refers to theprocess of generating an analyte ion having a net electrical chargeequal to one or more electron units. Negative ions are those ions havinga net negative charge of one or more electron units, while positive ionsare those ions having a net positive charge of one or more electronunits.

The term “desorption” as used herein refers to the removal of an analytefrom a surface and/or the entry of an analyte into a gaseous phase.

As used herein, the term “hemolysed” refers to the rupturing of the redblood cell membrane, which results in the release of hemoglobin andother cellular contents into the plasma or serum and the term “lipemic”refers to an excess of fats or lipids in blood.

As used herein, “liquid plasma” is plasma that is obtained from drawingblood from a patient and that is separated from the red blood cells butthat remains in a liquid state. Liquid plasma is generally obtained fromsubjects by phlebotomy or venipuncture.

As used herein, “dried plasma” is plasma that has been allowed to dry.Dried plasma may be produced following separation from red blood cellsby migration of the plasma through pores of a solid substrate (e.g., bylaminar flow) which restrict migration of cells as is described in moredetail herein.

As used herein, a “sampling paper” or “filter” or “filter membrane” or“laminar flow paper” or “laminar flow device” are terms usedinterchangeably to refer to a solid substrate for the collection ofdried blood and plasma may comprise a filter paper or membrane ontowhich blood can be spotted and that allows for the migration of theplasma away from the red blood cells to produce a region that issubstantially plasma and that upon drying, provides a sample of driedplasma.

As used herein, a “genotype” is the DNA sequence of the two allelespresent in a gene that may encode a protein sequence.

As used herein a “surrogate peptide” is a peptide derived from a proteinand that provides sequence information about the protein. As usedherein, the terms “variant specific surrogate peptide” and/or “allelespecific surrogate peptide” and/or “genotype specific surrogate peptide”is a peptide derived from a protein and that has a unique amino acidsequence directly attributable to the DNA sequence of the gene thatencodes for the protein. Determination of the sequence of the allelespecific surrogate peptide can be used to infer the genotype of at leastone allele of the gene. Thus, as used herein, an “allele specificsurrogate peptide” is a peptide that provides sequence information aboutthe allele which encodes the protein. For example, for ApoL1, awild-type surrogate peptide indicates that the protein is derived from awild-type allele, whereas a G1 surrogate peptide indicates that theprotein is derived from the G1 allele, and a G2 surrogate peptideindicates that the protein is derived from the G2 allele.

As used herein, a “common surrogate” peptide or “qualifying peptide” or‘qualifying surrogate peptide” is a peptide that has a unique amino acidsequence that does not vary when the DNA sequence at a locus of interestmay vary. Thus, as used herein, a “common surrogate peptide” or“qualifying peptide” comprises a peptide sequence that is common to thewild-type allele as well as all of the alleles being interrogated.Determination of the sequence of the common surrogate peptide will notvary with changes in the genotype at the locus of interest, and thus canbe used as internal controls to differentiate a true negative signalfrom a sample processing error.

As used herein, a “protein variant” is a protein that has an amino acidsequence that is different from the most common or wild-type sequence.

As used herein, a “proteomic profile” is a profile of surrogate peptidesthat can be used to determine the genotype of an individual at a locusof interest.

Analysis of APOL1 Genotypes by LC-MS/MS

Thus, embodiments of the present invention relate to methods and systemsfor proteomic analysis of genotypes. The present invention may beembodied in a variety of ways.

Methods for Analysis of Allele specific Proteomic Profiles by LC-MS/MS

In one embodiment, the present invention comprises a method fordetermining the genotype of a subject, the method comprising: providinga body fluid onto a substrate and drying; subjecting the dry body fluidto digestion to produce allele specific surrogate peptides from theprotein variant contained therein; using mass spectrometry to detectallele specific surrogate peptides present in the sample; anddetermining the genotype based on the proteomic profile of the allelespecific surrogate peptides. Thus, in an embodiment, provided is amethod for determining a genotype of a gene of interest in a subject,the method comprising: providing a body fluid from the subject, thebodily fluid containing a protein derived from the gene of interest;depositing the body fluid on a solid substrate, wherein the fluid isallowed to dry to produce a dry specimen; digesting the dry specimen togenerate at least one allele specific surrogate peptide for the protein;using mass spectrometry to detect the at least one allele specificsurrogate peptide present in the digested sample; and determining thegenotype of the subject based on the presence or absence or amount ofthe at least one allele specific surrogate peptide. In an embodiment,the method may further comprise measuring the amount of at least onecommon surrogate peptide that is common to each genotype of the gene ofinterest. Also, in an embodiment, the at least one allele specificsurrogate peptide is analyzed by liquid chromatography tandem massspectrometry (LC-MS/MS). In certain embodiments, the method may furthercomprise measuring the amount of the protein variant(s) coded by thegene alleles of interest by detection of the allele specific surrogatepeptides. In certain embodiments, the method may further comprisedetecting the presence of at least one common surrogate peptide that iscommon to all protein variants coded by the alleles of the gene ofinterest. Thus, in certain embodiments, the protease digestion mayproduce common surrogate peptides (or qualifying peptides) that arecommon to the various alleles present for the gene. In an embodiment,the presence or absence of the at least one allele-specific surrogatepeptide is determined by comparing a measured response for at least oneallele-specific surrogate peptide to a measured response for at leastone common surrogate peptide.

In some embodiments, digestion is performed with the protease trypsin.Or, other proteases and chemicals can be used for protein hydrolysis.Also, in some embodiments, the protein derived from the gene of interestis denatured prior to digestion to facilitate digestion.

In some cases an internal standard(s) may be used. The internal standardmay, in some embodiments, be added prior to the step of digestion. Forexample, the presence or absence of the at least one allele specificsurrogate peptide may be determined by comparing a measured response forthe at least one allele specific surrogate peptide to a measuredresponse for a stable isotope-labeled analogue of the at least oneallele specific surrogate peptide that is added to the sample.Additionally and/or alternatively, the presence or absence of the atleast one common surrogate peptide may be determined by comparing ameasured response for the common surrogate peptide to the response for astable isotope-labeled common surrogate peptide.

In an embodiment, fragment ions are produced for each of the peptidesduring MS/MS. Where the fragment ion is from the C-terminus, thefragment ion may be denoted “y”. Where the fragment ion is from theN-terminus, the fragment ion may be denoted “b”. Additionally, thefragment ion may be identified by the number of amino acid residues. Forexample, for a peptide having the sequence LNILNNNYK (SEQ ID NO. 4), thefragment ion y4 would have the sequence NNYK (SEQ ID NO. 13). In anembodiment, the results may be reported as the peak area ratio (PAR) forthe light (unlabeled) peptide (e.g., after trypsin digestion of theprotein) to the PAR for the heavy (stable isotope labeled) peptide. Thiscan provide a normalized response for the unlabeled tryptic peptide.

The method may be applied to any protein. In an embodiment, the proteinis ApoL1. For example, for ApoL1, the allele specific surrogate peptidemay have the amino acid sequence LNILNNNYK (SEQ ID NO. 4) derived fromthe wild-type allele (SEQ ID NO. 1), or may have the amino acid sequenceLNMLNNNYK (SEQ ID NO. 5) derived from the G1 allele (SEQ ID NO. 2), ormay have the amino acid sequence LNILNNK (SEQ ID NO. 6) derived from theG2 allele (SEQ ID NO. 3). Also for ApoL1, the common surrogate peptidemay have the amino acid sequence of SETAEELK (SEQ ID NO. 7) and/orVAQELEEK (SEQ ID NO. 8) wherein the common surrogate peptide is presentin each of the wild-type, G1 or G2 alleles. For the measurement of thesepeptides, the mass spectrometry may measure at least one of thetransitions in Table 3. Also, for ApoL1, the presence or absence of theat least one allele specific surrogate peptide may be determined bycomparing a measured response for the at least one allele specificsurrogate peptide to a measured response for a stable isotope-labeledanalogue listed in Table 2 of the at least one allele specific surrogatepeptide.

As disclosed herein, a variety of body fluids may be used. In somecases, the body fluid may be plasma or blood. In some embodiments, thebody fluid may be used to produce dried plasma. Or, the bodily fluid maycomprise at least one of dried blood, dried urine or dried saliva.

Also, in some embodiments, the sample may be subjected to a purificationstep prior to ionization for mass spectrometry. The purification stepmay comprise chromatography. As discussed herein, in certainembodiments, the chromatography comprises high performance liquidchromatography (HPLC). The LC step may comprise one LC separation, ormultiple LC separations. In one embodiment, the chromatographicseparation comprises extraction and analytical liquid chromatography.Additionally or alternatively, high turbulence liquid chromatography(HTLC) (also known as high throughput liquid chromatography) may beused.

The purification may comprise steps in addition to HPLC or other typesof chromatographic separation techniques. In alternate embodiments, themethod may comprise at least one of liquid-liquid extraction, supportedliquid extraction or dilution. In one embodiment, the sample is dilutedinto a solvent or solvent mixture that may be used for LC and/or MS(e.g., LC-MS/MS or 2D-LC-MS/MS).

As a non-limiting example, the methods of the disclosure have beenapplied to the proteomic analysis of ApoL1 genotypes. The wild-type (WT)allele and risk variant alleles (G1 and G2) code for ApoL1 which haveunique amino acid sequences at position 384 or 388-389 (FIG. 1). Upontryptic digestion of ApoL1 (e.g., FIGS. 1 and 2), each variant formconsequently produces a distinct proteolytic peptide derived fromresidues 382 to 390 (or 382 to 388 in the case of the G2 deletion),which can be detected by LC-MS/MS as a surrogate for presence of thecorresponding ApoL1 variant (FIG. 6 and FIG. 7). Thus, using the systemsand method disclosed herein, the three genetic variants ofApoL1—wild-type (WT), G1, and G2—may be determined by identifying thecorresponding mutations in the protein sequence of ApoL1 circulating inwhole blood by analysis of a dried blood or dried blood fraction (FIG.3).

FIG. 3 shows a substrate having zones of dried red blood cells and driedplasma isolated using a plasma separator strip compared to a paperhaving a dried blood spot, but that does not have the plasma separatedfrom the blood (see FIG. 3 left panel, top and bottom, respectively).The right side of FIG. 3 shows the extent of separation with time.Although dried blood spots are commonly used specimens in clinicalanalyses, the analysis may suffer from interferences derived from redblood cells (e.g., hemoglobin). Laminar flow paper may be used toseparate the plasma fraction of blood deposited from the cellularcomponents due to differential migration of the plasma and cells throughthe paper, resulting in a cell-free plasma fraction that can be driedand assayed.

The identification of the surrogate peptide(s) may be accomplished byfirst denaturing the dried blood fraction (i.e., dried plasma), followedby trypsin digestion to produce proteolytic surrogate peptides specificto the three variant forms of ApoL1 (FIG. 4). The digested plasma maythen directly analyzed by LC-MS/MS to determine the presence or absenceof the respective surrogate peptides to infer the presence or absence ofthe associated ApoL1 variant (FIG. 4 and FIG. 5).

Two common surrogate peptides (i.e., qualifying peptides) that arecommon among all three (i.e., WT, G1 and G2) ApoL1 variants can also bemonitored for qualifying sample processing (FIG. 2). For example, it ispossible another as of yet unknown ApoL1 variant may exist that codesfor a different amino acid mutation between residues 382 or 390. If anindividual is homozygous for this mutation (or heterozygous for two suchunknown mutations), this would result in no detectable variant-specificsurrogate peptide. In order to differentiate this from a sampleprocessing error, two additional surrogate peptides produced by trypsindigestion that neighbor the variant-specific surrogate peptide aremonitored, which should be detectable in all properly processed samplesregardless of the ApoL1 variant. Thus, detection of one or more of thesetwo “qualifying surrogate peptides” confirms proper sample processing,such that confident interpretation of the variant-specific surrogatepeptide detection may follow without concern for the integrity of theprocessed specimen.

The presence or absence of the surrogate peptides (variant orqualifying) may be determined by comparing the measured responses of thesurrogate peptide to the responses of its stable isotope-labeledanalogue that is added as an internal standard to sample aliquots priorto trypsin digestion. Based on the pattern of surrogate peptidesdetected, the genotype of the specimen/individual can be determined.

FIG. 4 shows a method for the determination of ApoL1 genotype usingdried plasma (3×¼″ ID round punches from a paper substrate) (e.g., asampling paper) in accordance with an embodiment of the disclosure. Thesample (i.e., dried plasma) may be added to digestion buffer. After ashort incubation at an elevated temperature (e.g., 30 min at 56° C.) todenature proteins present in the sample, trypsin and an internalstandard may be added and digestion is allowed to proceed (e.g., 30 minat 37° C.). Next formic acid can be added to terminate the trypsindigestion and precipitate acid-insoluble materials (i.e., deoxycholate),and an aliquot of the supernatant added to the LC-MS/MS system.

The samples may then be analyzed by high-throughput LC-MS/MS. FIG. 5shows a system for high-throughput proteomic genotyping. In oneembodiment, the LC method parameters were optimized to ensurechromatographic resolution of isobaric interferences of each surrogatepeptide, which resulted in an LC method having a total run-time of 3.5minutes. When employed on a multiplexing LC system, such as the ARITranscent™ TLX-4, injections may be run in parallel with injectionsstaggered every 1.5 minutes to improve the duty cycle of the massspectrometric analysis.

FIG. 6 shows surrogate peptide detection patterns for ApoL1 inaccordance with an embodiment of the disclosure. Thus, a subject who isa homozygous wild-type (WT/WT) will have the WT peptide, as well as thetwo common peptides (SETAEELK (SEQ ID NO. 7) and VAQELEEK (SEQ ID NO.8)). Similarly, a subject who is homozygous for either the G1 allele(G1/G1) or the G2 allele (G2/G2) will have the G1 or G2 peptide,respectively, as well as the two common peptides (SETAEELK (SEQ ID NO.7) and VAQELEEK (SEQ ID NO. 8)). Subjects who are heterozygous for WT,G1 or G2 will exhibit the patterns shown (FIG. 6).

FIG. 7 shows a qualitative assignment of ApoL1 genotypes in accordancewith an embodiment of the disclosure. Shown are example chromatogramsderived from individuals of all six potential genotypes. All fivesurrogate peptides are detected using two SRM transitions. When theApoL1 variant is present in the individual, the correspondingvariant-specific surrogate peptide is clearly visible/detected in thechromatogram. When the ApoL1 variant is absent in the individual, thecorresponding variant specific surrogate peptide is clearly notvisible/detected in the chromatogram. Notably, the two qualifyingsurrogate peptides are present in all six individuals regardless ofgenotype.

FIG. 8 shows a semi-quantitative assessment of ApoL1 genotypes inaccordance with an embodiment of the disclosure. In this experiment,several hundred specimens and specimen pools were analyzed as liquidplasma and qualitatively assigned as negative (triangles/diamonds) orpositive (circles) for a given variant-specific surrogate peptide basedon visual interpretation of the corresponding chromatograms. Based onthe normalized response for each transition shown on the y axis as apeak area ratio for fragments (e.g. y₆+ or y₅+ for the wild-type allelespecific peptide) of the unlabeled allele specific peptide as comparedto the heavy isotope labeled allele specific peptide (light: heavypeptide peak area ratio, PAR) measured in the associated negativespecimens, the limit of detection was calculated as the mean normalizedresponse, plus four (4) standard deviations, below which a sample wouldbe classified as definitively negative. The threshold for definitivepositive detection of the variant-specific surrogate peptide wascalculated as the mean normalized response in the associated negativespecimens, plus 12 standard deviations.

FIG. 9 shows a comparison of DNA sequencing and proteomic genotypingusing dried plasma vs liquid plasma biological samples. Matched liquidand dry plasma specimens were obtained from 209 African American donorsfor proteomic analysis, along with whole blood for Sanger sequencing.Genotypes of each donor determined by Sanger sequencing was in perfectagreement with the genotypes assigned by proteomic analysis from bothliquid and dry plasma specimens indicating that proteomic profiling is aviable option for LC-MS/MS/analysis of biomarkers of interest.

FIG. 10 shows an assessment of the ApoL1 wild-type (WT) peptide as afunction of hemoglobin concentration in accordance with an embodiment ofthe disclosure. In some embodiments, hemoglobin may interfere with thedetection of a biomarker of interest in two significant ways. First, athigher concentrations of hemoglobin an isobaric interferent was observedin one of two SRM transitions for the WT-surrogate peptide, whichconfounded interpretation of specimens with WT-negative genotypes (i.e.,G1/G1, G2/G2 or GI/G2). Second, a matrix effect was observed whichresulted in lower analytical response due to ion suppression.

FIG. 11 shows a titration of ApoL1 WT, G1 and G2 peptides as a functionof dried plasma immobilized on a filter paper and harvested from ¼ inchpunches in accordance with an embodiment of the disclosure. Thecross-section and number of dry plasma punches required to provideequivalent analytical response to liquid plasma was evaluated to ensureequivalent assay performance from both liquid and dry plasma. Differentcross-sections were considered to determine which was most practical forimplementation in a 96-well plate format. Three individuals wereevaluated with the following genotypes: WT/WT, WT/G1, and WT/G2.Relative to the liquid plasma, the relative response of each surrogatepeptide was consistent across individuals and genotypes indicatingresponse recovery in dry plasma was related solely to the total surfacearea analyzed. Based on these analyses, it was concluded that anequivalent analytical response of 20 uL liquid plasma could be obtainedfrom between 96 and 126 mm² of dry plasma for the wild-type peptide(WT), as well as G1 and G2. The WT graph shows examples of other cut-outshapes that may be used. Signal is generally proportional to the amountof cut-out used per sample.

FIG. 12 shows a trypsin digestion time course using either liquid plasmaor dried plasma in accordance with an embodiment of the disclosure. Theformation of each surrogate peptide during digestion of liquid and dryplasm was evaluated in a time course analysis between 0 and 120 minutes.Samples of the liquid plasma digestion were collected at nine timepoints between 3 and 120 minutes. The profiles indicate digestion ofeach ApoL1 variant proceeds in a similar manner from both liquid and dryplasma specimens. Thus, there is virtually 100% digestion after about 2hours.

FIG. 13 shows stability of protein variant measurements from dry plasmain accordance with an embodiment of the disclosure. It can be seen thatthe samples provide similar measured responses of the surrogate peptideseven after 28 days at either 23 degrees Centigrade (the normal storageconditions) or 37 degrees Centigrade.

FIG. 14 shows a schematic ellustration of an embodiment of a system ofthe present invention.

Systems for LC-MS/MS Proteomic Genotyping

Other disclosed embodiments comprise systems. For example, disclosed isa system for determining the proteomic profile for genotype of interestin a subject, the system comprising: a device for providing a testsample comprising a protein or allele specific surrogate peptidesderived from the genetic locus of interest; a station for subjecting thesample to protease digestion protease to generate the allele specificsurrogate peptides; optionally, a station for chromatographicpurification of the allele specific surrogate peptide(s); and a stationfor analyzing the allele specific surrogate peptide(s) by massspectrometry to determine the presence or amount of the allele specificsurrogate peptides in the biological sample. In an embodiment, disclosedis a system for determining the genotype of a gene of interest in asubject, the system comprising: a device for providing a dried bodyfluid comprising a protein derived from the gene of interest; a stationfor subjecting the dried body fluid to digestion to generate at leastone allele specific surrogate peptide and optionally, at least onecommon surrogate peptide for the protein; optionally, a station forchromatographic purification of the at least one allele specificsurrogate peptide; and a station for analyzing the at least one allelespecific surrogate peptide by mass spectrometry to determine thepresence or amount of the at least one allele specific surrogate peptidein the biological sample. In an embodiment, the protein is ApoL1.

In certain embodiments, the device for providing a biological sample maycomprise a device to immobilize and separate red blood cells from plasmaon a substrate. The system may further comprise a station for adding astable isotope labeled internal standard for the at least one allelespecific surrogate peptide and optionally, the at least one commonsurrogate peptide for the protein.

Also, as described in detail below, the station for mass spectrometrymay comprise a tandem mass spectrometer. In an embodiment, the massspectrometry is operated in Electrospray Ionization (ESI) mode.

Also, the station for chromatography may comprise various types ofchromatography separately or used together such as, but not limited to,liquid-liquid chromatography, and/or high performance liquidchromatography (HPLC), and/or other types of chromatography describedherein.

Also in certain embodiments, at least one of the stations is automatedand/or controlled by a computer. For example, as described herein, incertain embodiments, at least some of the steps are automated such thatlittle to no manual intervention is required.

In one embodiment, the station for chromatographic separation comprisesat least one apparatus to perform liquid chromatography (LC). In oneembodiment, the station for liquid chromatography comprises a column forextraction chromatography. Additionally or alternatively, the stationfor liquid chromatography comprises a column for analyticalchromatography. In certain embodiments, the column for extractionchromatography and analytical chromatography comprise a single stationor single column. For example, in one embodiment, liquid chromatographyis used to purify the biomarker of interest from other components in thesample that co-purify with the biomarker of interest after extraction ordilution of the sample.

The system may also include a station for analyzing thechromatographically separated one or more biomarkers of interest by massspectrometry to determine the presence or amount of the one or morebiomarkers in the test sample. In certain embodiments, tandem massspectrometry is used (MS/MS). For example, in certain embodiments, thestation for tandem mass spectrometry comprises an Applied BiosystemsAPI5500 or API6500 triple quadrupole or thermo Q-Exactive massspectrometer.

The system may also comprise a station for partially purifying ordenaturing peptides and/or proteins from the biological sample and/ordiluting the sample. In an embodiment, the station for extractioncomprises a station for immunoaffinity enrichment of the protein variantor resulting surrogate peptide. The station for immunoaffinityenrichment may comprise equipment and reagents for manipulation,washing, and stripping of the solid sorbent binding the immunoaffinityreagent. In some cases an isotopically-labeled internal standard is usedto standardize losses of the biomarker that may occur during theprocedures.

In certain embodiments, the methods and systems of the present inventionmay comprise multiple liquid chromatography steps. Thus, in certainembodiments, a two-dimensional liquid chromatography (LC) procedure isused. For example, in one embodiment, the method and systems of thepresent invention may comprise transferring the sample, or peptidesderived from the sample, from a LC extraction column to an analyticalcolumn. In one embodiment, the transferring from the extraction columnto an analytical column is done by a heart-cutting technique. In anotherembodiment, transfer from the extraction column to an analytical columnby a chromatofocusing technique. Alternatively, transfer from theextraction column to an analytical column may be done by a columnswitching technique. These transfer steps may be done manually, or maybe part of an on-line system.

Various columns comprising stationary phases and mobile phases that maybe used for extraction or analytical liquid chromatography are describedherein. The column used for extraction liquid chromatography may bevaried depending on the biomarker of interest. In some embodiments, theextraction column is a functionalized silica or polymer-silica hybrid orpolymeric particle or monolithic silica stationary phase, such as aPoroshell SBC-18 column. The column used for analytical liquidchromatography may be varied depending on the analyte and/or the columnthat was used for the extraction liquid chromatography step. Forexample, in certain embodiments, the analytical column comprisesparticles having an average diameter of about 5 μm.

As noted herein, in certain embodiments, the mass spectrometer maycomprise a tandem mass spectrometer (MS/MS). For example, in oneembodiment of the methods and systems of the present invention, thetandem mass spectrometry comprises a triple quadrupole tandem massspectrometer. In other embodiments, the tandem mass spectrometer may bea hybrid mass spectrometer, such as a quadrupole-oribtrap or aquadrupole-time-of-flight mass spectrometer.

The tandem MS/MS may be operated in a variety of modes. In oneembodiment, the tandem MS/MS spectrometer is operated in an ElectrosprayIonization (ESI) mode. In some embodiments, the quantification of theanalytes and internal standards is performed in the selected reactionmonitoring mode (SRM).

The systems and methods of the present invention may, in certainembodiments, provide for a multiplexed or high throughput assay. Forexample, certain embodiments of the present invention may comprise amultiplexed liquid chromatography tandem mass spectrometry (LC-MS/MS) ortwo-dimensional or tandem liquid chromatography-tandem mass spectrometry(LC)-LC-MS/MS) methods for the proteomic analysis.

In some embodiments, a tandem MS/MS system is used. As is known by thoseof skill in the art, in tandem MS spectrometry, the precursor ion isselected following ionization, and that precursor ion is subjected tofragmentation to generate product (i.e., fragment) ions, whereby one ormore product ions are subjected to a second stage of mass analysis fordetection. A sample may therefore be analyzed for peptides thatcorrespond to more than one genotype (i.e., for ApoL1, the WT, G1, G2and common peptides) since the peptides have different precursor andproduct ions in tandem mass spectrometric methodologies (i.e., differenttransitions).

The analyte of interest may then be detected based upon the amount ofthe characteristic transitions measured by tandem MS. In someembodiments, the tandem mass spectrometer comprises a triple quadrupolemass spectrometer. In some embodiments, the tandem mass spectrometer isoperated in a positive ion Electrospray Ionization (ESI) mode. Or, othermethods of ionization such as Matrix Assisted LaserDesorption/Ionization (MALDI) may be used for ionization. In someembodiments, the detection of the analytes and internal standards isperformed in the selected reaction monitoring mode (SRM).

In other embodiments, a surrogate peptide, labeled with a heavy stableisotopes is added to the sample at an appropriate point in the procedure(e.g., prior to digestion) to correct for incomplete digestion and/orsample loss at any step.

The temperature for heating the sample during ionization may, inalternate embodiments range from 100° C. to about 1000° C. and includesall ranges therein. In an embodiment, a dehydration step is performedwithin the interface of the mass spectrometer employed in electrospraymode at 500 degrees C.±100 degrees. In an embodiment, the sample isheated for several microseconds at the interface for dehydration tooccur. In alternate embodiments, the heating step is done for less than1 second, or less than 100 milliseconds (msec), or less than 10 msec, orless than 1 msec, or less than 0.1 msec, or less than 0.01 msec, or lessthan 0.001 msec.

FIG. 14 shows an embodiment of a system of the present invention. Asshown in FIG. 14, the system may comprise a station for processing asample (104) that may comprise a biomarker of interest into samplingcontainers (e.g., 96 well microtiter assay wells). In one embodiment,the sample is aliquoted into a container or containers to facilitateprotease digestion and/or enrichment and/or sample dilution. The stationfor aliquoting may comprise receptacles to discard the portion of thebiological sample that is not used in the analysis.

The system may further comprise a station for adding an internalstandard to the sample (108). In an embodiment, the internal standardcomprises the biomarker of interest labeled with a heavy, stableisotope. Thus, the station for adding an internal standard may comprisesafety features to facilitate adding an isotopically labeled internalstandard solutions to the sample. The system may also, in someembodiments, comprise a station (110) for enrichment, proteinprecipitation and/or dilution of the sample.

The system may also comprise a station for liquid chromatography (LC) ofthe sample. As described herein, in an embodiment, the station forliquid chromatography may comprise an extraction liquid chromatographycolumn (112). The station for liquid chromatography may comprise acolumn comprising the stationary phase, as well as containers orreceptacles comprising solvents that are used as the mobile phase. In anembodiment, the mobile phase comprises a gradient of methanol and water,acetonitrile and water, or other miscible solvents with aqueous volatilebuffer solutions. Thus, in one embodiment, the station may comprise theappropriate lines and valves to adjust the amounts of individualsolvents being applied to the column or columns. Also, the station maycomprise a means to remove and discard those fractions from the LC thatdo not comprise the biomarker of interest. In an embodiment, thefractions that do not contain the biomarker of interest are continuouslyremoved from the column and sent to a waste receptacle fordecontamination and to be discarded.

The system may also comprise an analytical LC column (114). Theanalytical column may facilitate further purification and concentrationof the biomarker of interest as may be required for furthercharacterization and quantification.

Also, the system may comprise a station for characterization andquantification of the allele specific surrogate peptide. In oneembodiment, the system may comprise a station for in source ionization(115) and a station for mass spectrometry (MS) (116) of the biomarker.In an embodiment, the station for mass spectrometry comprises a stationfor tandem mass spectrometry (MS/MS). Also, the station forcharacterization and quantification may comprise a station for dataanalysis (118) and/or a computer (102) and software for analysis of theMS/MS results. In an embodiment, the analysis comprises bothidentification and quantification of the allele specific surrogatepeptide.

In some embodiments, one or more of the purification or separation stepscan be performed “on-line.” As used herein, the term “on-line” refers topurification or separation steps that are performed in such a way thatthe test sample is disposed, e.g., injected, into a system in which thevarious components of the system are operationally connected and, insome embodiments, in fluid communication with one another. The on-linesystem may comprise an autosampler for removing aliquots of the samplefrom one container and transferring such aliquots into anothercontainer. For example, an autosampler may be used to transfer thesample after extraction onto an LC extraction column. Additionally oralternatively, the on-line system may comprise one or more injectionports for injecting the fractions isolated from the LC extractioncolumns onto the LC analytical column. Additionally or alternatively,the on-line system may comprise one or more injection ports forinjecting the LC purified sample into the MS system. Thus, the on-linesystem may comprise one or more columns, including but not limited to,an extraction column, including an HTLC extraction column, and in someembodiments, an analytical column. Additionally or alternatively, thesystem may comprise a detection system, e.g., a mass spectrometersystem. The on-line system may also comprise one or more pumps; one ormore valves; and necessary plumbing. In such “on-line” systems, the testsample and/or analytes of interest can be passed from one component ofthe system to another without exiting the system, e.g., without havingto be collected and then disposed into another component of the system.

In some embodiments, the on-line purification or separation method canbe automated. In such embodiments, the steps can be performed withoutthe need for operator intervention once the process is set-up andinitiated. Thus, in various embodiments, the system, or portions of thesystem may be controlled by a computer or computers (102). Thus, incertain embodiments, the present invention may comprise software forcontrolling the various components of the system, including pumps,valves, autosamplers, and the like. Such software can be used tooptimize the extraction process through the precise timing of sample andsolute additions and flow rate.

Although some or all of the steps in the method and the stationscomprising the system may be on-line, in certain embodiments, some orall of the steps may be performed “off-line.” In contrast to the term“on-line”, the term “off-line” refers to a purification, separation, orextraction procedure that is performed separately from previous and/orsubsequent purification or separation steps and/or analysis steps. Insuch off-line procedures, the analytes of interests typically areseparated, for example, on an extraction column or by liquid/liquidextraction, from the other components in the sample matrix and thencollected for subsequent introduction into another chromatographic ordetector system. Off-line procedures typically require manualintervention on the part of the operator.

Liquid chromatography may, in certain embodiments, comprise highturbulence liquid chromatography or high throughput liquidchromatography (HTLC). See, e.g., Zimmer et al., J. Chromatogr. A854:23-35 (1999); see also, U.S. Pat. Nos. 5,968,367; 5,919,368;5,795,469; and 5,772,874. Traditional HPLC analysis relies on columnpackings in which laminar flow of the sample through the column is thebasis for separation of the analyte of interest from the sample. In suchcolumns, separation is a diffusional process. Turbulent flow, such asthat provided by HTLC columns and methods, may enhance the rate of masstransfer, improving the separation characteristics provided. In someembodiments, high turbulence liquid chromatography (HTLC), alone or incombination with one or more purification methods, may be used to purifythe biomarker of interest prior to mass spectrometry. In suchembodiments, samples may be extracted using an HTLC extraction cartridgewhich captures the analyte, then eluted and chromatographed on a secondHTLC column or onto an analytical HPLC column prior to ionization.Because the steps involved in these chromatography procedures can belinked in an automated fashion, the requirement for operator involvementduring the purification of the analyte can be minimized. Also, in someembodiments, the use of a high turbulence liquid chromatography samplepreparation method can eliminate the need for other sample preparationmethods including liquid-liquid extraction. Thus, in some embodiments,the test sample, e.g., a biological fluid, can be disposed, e.g.,injected, directly onto a high turbulence liquid chromatography system.

For example, in a typical high turbulence or turbulent liquidchromatography system, the sample may be injected directly onto a narrow(e.g., 0.5 mm to 2 mm internal diameter by 20 to 50 mm long) columnpacked with large (e.g., >25 micron) particles. When a flow rate (e.g.,3-500 mL per minute) is applied to the column, the relatively narrowwidth of the column causes an increase in the velocity of the mobilephase. The large particles present in the column can prevent theincreased velocity from causing back pressure and promote the formationof vacillating eddies between the particles, thereby creating turbulencewithin the column.

In high turbulence liquid chromatography, the analyte molecules may bindquickly to the particles and typically do not spread out, or diffuse,along the length of the column. This lessened longitudinal diffusiontypically provides better, and more rapid, separation of the analytes ofinterest from the sample matrix. Further, the turbulence within thecolumn reduces the friction on molecules that typically occurs as theytravel past the particles. For example, in traditional HPLC, themolecules traveling closest to the particle move along the column moreslowly than those flowing through the center of the path between theparticles. This difference in flow rate causes the analyte molecules tospread out along the length of the column. When turbulence is introducedinto a column, the friction on the molecules from the particle isnegligible, reducing longitudinal diffusion.

The methods and systems of the present invention may use massspectrometry to detect and quantify the biomarker of interest. The terms“mass spectrometry” or “MS” as used herein generally refer to methods offiltering, detecting, and measuring ions based on their mass-to-chargeratio, or “m/z.” In MS techniques, one or more molecules of interest areionized, and the ions are subsequently introduced into a massspectrometer where, due to a combination of electric fields, the ionsfollow a path in space that is dependent upon mass (“m”) and charge(“z”).

In certain embodiments, the mass spectrometer uses a “quadrupole”system. In a “quadrupole” or “quadrupole ion trap” mass spectrometer,ions in an oscillating radio frequency (RF) field experience a forceproportional to the direct current (DC) potential applied betweenelectrodes, the amplitude of the RF signal, and m/z. The voltage andamplitude can be selected so that only ions having a particular m/ztravel the length of the quadrupole, while all other ions are deflected.Thus, quadrupole instruments can act as both a “mass filter” and as a“mass detector” for the ions injected into the instrument.

In certain embodiments, tandem mass spectrometry is used. See, e.g.,U.S. Pat. No. 6,107,623, entitled “Methods and Apparatus for Tandem MassSpectrometry,” which is hereby incorporated by reference in itsentirety. Further, the selectivity of the MS technique can be enhancedby using “tandem mass spectrometry,” or “MS/MS.” Tandem massspectrometry (MS/MS) is the name given to a group of mass spectrometricmethods wherein “parent or precursor” ions generated from a sample arefragmented to yield one or more “fragment or product” ions, which aresubsequently mass analyzed by a second MS procedure. MS/MS methods areuseful for the analysis of complex mixtures, especially biologicalsamples, in part because the selectivity of MS/MS can minimize the needfor extensive sample clean-up prior to analysis. In an example of anMS/MS method, precursor ions are generated from a sample and passedthrough a first mass filter to select those ions having a particularmass-to-charge ratio. These ions are then fragmented, typically bycollisions with neutral gas molecules in a suitable ion containmentdevice, to yield product (fragment) ions, the mass spectrum of which isrecorded by an electron multiplier detector. The product ion spectra soproduced are indicative of the structure of the precursor ion, and thetwo stages of mass filtering can eliminate ions from interfering speciespresent in the conventional mass spectrum of a complex mixture.

In an embodiment, the methods and systems of the present invention use atriple quadrupole MS/MS (see e.g., Yost, Enke in Ch. 8 of Tandem MassSpectrometry, Ed. McLafferty, pub. John Wiley and Sons, 1983). Triplequadrupole MS/MS instruments typically consist of two quadrupole massfilters separated by a fragmentation means. In one embodiment, theinstrument may comprise a quadrupole mass filter operated in the RF onlymode as an ion containment or transmission device. In an embodiment, thequadrupole may further comprise a collision gas at a pressure of between1 and 10 millitorr. Many other types of “hybrid” tandem massspectrometers are also known, and can be used in the methods and systemsof the present invention including various combinations of orbitrapanalyzers and quadrupole filters. These hybrid instruments oftencomprise high resolution orbitrap analyzers (see e.g., Hu Q, Noll R J,Li H, Makarov A, Hardman M, Graham Cooks R. The Orbitrap: a new massspectrometer. J Mass Spectrom. 2005; 40(4):430-443) for the second stageof mass analysis. Use of high resolution mass analyzer may be highlyeffective in reducing chemical noise to very low levels.

For the methods and systems of the present invention, ions can beproduced using a variety of methods including, but not limited to,electron ionization, chemical ionization, fast atom bombardment, fielddesorption, and matrix-assisted laser desorption ionization (“MALDI”),surface enhanced laser desorption ionization (“SELDI”), photonionization, electrospray ionization, and inductively coupled plasma.

In those embodiments, such as MS/MS, where precursor ions are isolatedfor further fragmentation, collision-induced dissociation (“CID”) may beused to generate the fragment ions for further detection. In CID,precursor ions gain energy through collisions with an inert gas, andsubsequently fragment by a process referred to as “unimoleculardecomposition.” Sufficient energy must be deposited in the precursor ionso that certain bonds within the ion can be broken due to increasedvibrational energy.

In some embodiments, to attain the required analytical selectivity andsensitivity, the presently disclosed 2D-LC-MS/MS methods includemultiplexed sample preparation procedures. For example, in certainembodiments dialysis of the sample is performed using a 96 well platehaving a dialysis membrane in each well or multiple sample tubes.Additionally or alternatively, the multiplex system may comprisestaggered multiplexed LC and MS sample inlet systems. Also, the methodsand systems of the present invention may comprise multiple columnswitching protocols, and/or heart-cutting (LC-LC or 2D-LC) techniques,and/or LC separations prior to MS detection. In some embodiments, themethods and systems of the present invention may include a multiplexedtwo-dimensional liquid chromatographic system coupled with a tandem massspectrometer (MS/MS) system, for example a triple quadrupole MS/MSsystem. Such embodiments provide for staggered, parallel sample inputinto the MS system.

Thus, multiple samples may each be applied to individual extractioncolumns. Once the samples have each run through the extraction column,they may each be transferred directly (e.g., by column switching) to asecond set of analytical columns. As each sample elutes from theanalytical column, it may be transferred to the mass spectrometer foridentification and quantification.

A plurality of analytes can be analyzed simultaneously or sequentiallyby the presently disclosed LC-MS/MS and 2D-LC-MS/MS methods. Exemplaryanalytes amenable to analysis by the presently disclosed methodsinclude, but are not limited to, peptides, steroid hormones, nucleicacids, vitamins and the like. One of ordinary skill in the art wouldrecognize after a review of the presently disclosed subject matter thatother similar analytes could be analyzed by the methods and systemsdisclosed herein. Thus, in alternate embodiments, the methods andsystems may be used to quantify steroid hormones, protein and peptidehormones, peptide and protein biomarkers, drugs of abuse and therapeuticdrugs. For example, optimization of key parameters for each analyte canbe performed using a modular method development strategy to providehighly tuned bioanalytical assays. Thus, certain steps may be varieddepending upon the analyte being measured as disclosed herein.

Also, embodiments of the methods and systems of the present inventionmay provide equivalent sensitivity attainable for many of the analytesbeing measured using much less sample. For example, through using thisoptimization procedure, an LLOQ of about 10 nanomoles/L of ApoL1 fordried plasma corresponding to about 20 μL of liquid plasma. Such smallsample sizes render sampling (often by finger-prick) much moreaccessible.

EXAMPLES

Additional data from the analytical validation and standard operatingprocedures for the presently disclosed method are set forth in thefollowing Examples.

The following Examples have been included to provide guidance to one ofordinary skill in the art for practicing representative embodiments ofthe presently disclosed subject matter. In light of the presentdisclosure and the general level of skill in the art, those of skill canappreciate that the following Examples are intended to be exemplary onlyand that numerous changes, modifications, and alterations can beemployed without departing from the scope of the presently disclosedsubject matter.

Example 1 LC-MS/MS for ApoL1 Genotyping

Two genetic variants of ApoL1, termed G1 and G2, as well as wild-typealleles were measured by LC-MS/MS.

In addition to standard serum and plasma specimens, the genetic testutilizes specimens acquired on blood collection devices that depositblood on a substrate. These collection devices provide a meteringmechanism that spots a defined volume of blood onto plasma separationstrips. This automation is intended to provide an easier samplingmechanism for the patient. Dried blood is an alternate specimencollection process that utilizes a finger stick and a plasma separatorstrip instead of venipuncture collection of serum or plasma tubes. Thefunctional core of the collection strip is a specialized blood separatormaterial that restricts the migration of cells from the application sitewhile allowing the lateral flow of plasma. This selective migrationseparates the cells and plasma within the lateral flow material similarto the separation obtained from the centrifugation of a serum separatortube. Dried plasma from a standardized punched section of the separationmaterial can be analyzed in place of liquid plasma or serum usingestablished laboratory procedures.

The three genetic variants of ApoL1—wild-type (WT), G1, and G2—weredetermined by identifying the corresponding mutations in the proteinsequence of ApoL1 circulating in whole blood. This was accomplished byfirst denaturing the serum or plasma sample, followed by trypsindigestion to produce proteolytic surrogate peptides specific to thethree variant forms of ApoL1. The digested plasma was then directlyanalyzed by LC-MS/MS to determine the presence or absence of therespective surrogate peptides to infer the presence or absence of theassociated ApoL1 variant. Two surrogate peptides common among all threeApoL1 variants were also monitored for qualifying sample processing. Thepresence or absence of the surrogate peptides was determined bycomparing the measured responses of the surrogate peptide to theresponses of its stable isotope-labeled analogue that was added as aninternal standard to sample aliquots prior to trypsin digestion. Basedon the pattern of surrogate peptides detected, the genotype of thespecimen/individual was determined.

Assay Summary and Surrogate Peptide Specificity

TABLE 1 Surrogate Peptide Specificity Analyte Protein Name:Human Apolipoprotein L1 Protein Database Reviewed:UniProt; Homo sapiens (canonical + isoforms);UP000005640, accessed 2018 Feb. 27 Analyte Protein Accession(s): O14791Protease (specificity): Trypsin (R/K|P) Conserved, QualifyingSETAEELK (SEQ ID NO. 7), aa 365-371 Surrogate PeptideVAQELEEK (SEQ ID NO. 8), aa 373-380 Sequence(s): Variant ProteinWT (wild-type) G1* G2 Nomenclature: DNA Sequence Mutation: — 1152T>G1169delTTATAA Protein Sequence Mutation: — Ile384Met N388_Y389delVariant-specific Surrogate LNILNNNYK LNMLNNNYK LNILNNKPeptide Sequences: (SEQ ID NO. 4) (SEQ ID NO. 5) (SEQ ID NO. 6)aa 382-390 aa 382-390 aa 382-388 Surrogate Peptideno annotated PTMs, no annotated sequence  Modifications: variantsSurrogate Peptide BLAST: specific to ApoL1, no close alignments require specificity testing *The G1 risk allele is comprised of twosingle nucleotide variants 1024A>G (Ser342Gly) and 1152T>G (Ile384Met)which are in near perfect disequilibrium (i.e. occur together in thevast majority of the time). This LC-MS/MS assay does not test for thepresence of 1024A>G (Ser342Gly).

Stock Internal Standard Solutions

Stock internal standard material of the following synthetic, stableisotopically-labeled peptides were purchased commercially (New EnglandPeptide) as 0.1 mg (100 μg, net) dry aliquots with amino acid analysis.

TABLE 2 Internal Standard Peptides Abbre- SEQ viation ID No. SequenceLabel(s) SET 7 SETAEEL{circumflex over ( )}K{circumflex over ( )}L{circumflex over ( )} = (SEQ ID NO. 14) [15N, 13C6]- VAQ 8VAQEL{circumflex over ( )}EEK{circumflex over ( )} Leucine(SEQ ID NO. 15) K{circumflex over ( )} = LNI_WT 4 L{circumflex over( )}NILNNNYK{circumflex over ( )} [15N2, 13C6]- (SEQ ID NO. 16) LysineLNM_G1 5 L{circumflex over ( )}NMLNNNYK{circumflex over ( )}(SEQ ID NO. 17) LNI_G2 6 L{circumflex over ( )}NILNNK{circumflex over( )} (SEQ ID NO. 18)

Individual stock internal standard solutions were prepared by adding 0.4mL of 0.001% zwittergent 3-16 with 0.1% formic acid directly to a single0.1 mg vial produce a ˜200 μg/mL solution. These internal standard stocksolutions were allowed to incubate for 15 minutes prior to use and ifnot used within 4 hours, frozen at −70° C.

Prior to use, the exact concentration of each stock solution wasassigned by UV absorbance using a NanoDrop™ 2000c Spectrophotometer at205 nm, with baseline correction at 340 nm. Each stock was measured onthe NanoDrop™ pedestal as at least 10 replicates following blanking with0.001% zwittergent 3-16 with 0.1% formic acid. The mean absorbance(A₂₀₅) should be between 0.3 and 1.2, with a CV less than 3%, and wasused to calculate the stock concentration (C_(stock)) based on a pathlength (b) of 0.1 cm and extinction coefficient (ε₂₀₅) of 0.031 mL/μg/cmaccording to the equation below:

$C_{stock} = {\frac{A_{205}}{ɛ_{205} \times b}.}$

Control Preparation

Negative Control—the negative control was 30 mg/mL human serum albumin(HSA).

Positive Control The positive control was a pool of male plasma (GoldenWest Biologicals, Cat # MSG15000M). This control served as positivecontrol for the WT variant and as a “weak positive” control for both theG1 and G2 variants.

WT/G1 EDTA Plasma Control A EDTA plasma specimens from WT/WT, G1/G1, andWT/G1 individuals were pooled to ensure a balanced ratio of G1 and WTvariants. This control served as a positive control for both the WT andG1 variants, while serving as a negative control for the G2 variant.

WT/G2 EDTA Plasma Control B EDTA plasma specimens from WT/WT, G2/G2, andWT/G2 individuals were pooled to ensure a balanced ratio of G2 and WTvariants. This control served as a positive control for both the WT andG2 variants, while serving as a negative control for the G1 variant.

G1/G2 EDTA Plasma Control C EDTA plasma specimens from G1/G1, G2/G2, andG1/G2 individuals were pooled to ensure a balanced ratio of G1 and G2variants. This control served as a positive control for both the G1 andG2 variants, while serving as a negative control for the WT variant. Atleast one replicate of each of the following whole blood QC samples wasincluded in each of the first 20 batches used to establish inter-assayreproducibility as dry punches produced at least 2 hours followingdeposition of 180 μL onto a lateral flow substrate for separation ofdried blood cells from plasma.

WT/G1 LiHep Whole Blood Control A Lithium heparin whole blood specimensfrom WT/WT, G1/G1, and WT/G1 individuals were pooled to ensure abalanced ratio of G1 and WT variants. This control will served as apositive control for both the WT and G1 variants, while serving as anegative control for the G2 variant.

WT/G2 LiHep Whole Blood Control B Lithium heparin specimens from WT/WT,G2/G2, and WT/G2 individuals were pooled to ensure a balanced ratio ofG2 and WT variants. This control served as a positive control for boththe WT and G2 variants, while serving as a negative control for the G1variant.

G1/G2 LiHep Whole Blood Control C Lithium heparin whole blood specimensfrom G1/G1, G2/G2, and G1/G2 individuals were pooled to ensure abalanced ratio of G1 and G2 variants. This control will serve as apositive control for both the G1 and G2 variants, while serving as anegative control for the WT variant.

Assay Procedure

Controls, samples, digestion buffer, and working internal standards (IS)were thawed at room temperature (20-25° C.). Aliquots (e.g., 20 μL) ofcontrols and samples were pipetted into wells in a 2 mL 96-deep wellplate. The negative control received two aliquots. For the dried plasmasamples, generally three (3) ¼″ diameter punches of dry plasma from thelateral flow substrate were used in place of 20 μL liquid sample. Next,180 μL digestion buffer (50 mM Tris-HCl, 0.675 mM DTT, 6.75 mg/mL DOC,pH 8.0) was added into each of the wells. The plate wells were thensealed, and after centrifugation for 5-30 seconds, the plate wasincubated on the Thermomixer at 56° C. and 1500 rpm for 30 minutes (toallow for denaturation of the proteins and extraction of the driedplasma to occur).

At this point, the internal standards for each of the peptides wereadded into each well (except for the double negative control whichreceived 20 μL 0.001% Zwittergent 3-16). An aliquot (25 μL) of trypsinsolution (32 mg/mL Tryp sin in 50 mM Acetic Acid) was added to each welland after sealing each of the wells, the plate was centrifuged for 5-30seconds, then incubated on the Thermomixer at 37° C. and 1500 rpm for 30minutes to allow digestion to occur. After digestion is complete (30min) 1000 μL of Quench Buffer (0.001% (w/v) Zwittergent 3-16 with 2%formic acid) is added into each well. The wells were then sealed withfoil and the samples mixed for 1 min at 1500-3500 rpm. Aftercentrifugation (10 min at 3500 rpm) 200 uL of supernatant wastransferred into wells in a new 1 mL 96-deep well plate and the samplesprocessed for LC-MS/MS.

HPLC-MS/MS

HPLC was performed using an Aria Transcend TX4 System (Thermo-Fischer)consisting of 8 1200SL Series Binary Pumps and 4 1200 Series VacuumDegasser and employed a gradient of formic acid in water andacetonitrile. Selected Reaction Monitoring (SRM, i.e., MS/MS) employed aAPI 5500 Tandem Mass Spectrometer (Sciex, Toronto Canada) and Turbo V™Ion Source with Electrospray. SRM transitions for the various fragmentions generated are shown below in Table 3. For example, for unlabeledQualifying Peptide 1, the transition of 453.724→690.367 was measured asthe primary transition and the transition of 453.724→217.082 m/z wasmeasured as the secondary transition. As larger molecules (like proteinsand peptides) often have more than 1 charge (typically 2 or 3 for atryptic peptide), it is not uncommon for peptides in Q1 to have asmaller mass (m) to charge (z) ratio (i.e., where z=2), but then lose acharge during fragmentation such that Q3 isolates a larger m/z (i.e.,where z=1).

TABLE 3 SRM Transitions Q1 Mass Q3 Mass Dwell Primary or (Da) (Da)(msec) Param Value ID Secondary 453.724 690.367 50 DP 64.2sp|O14791|APOL1_WT.SETAEELK.+2y6.light primary CE 20.2 453.724 217.082 5 DP 64.2 sp|O14791|APOL1_WT.SETAEELK.+2b2.light secondary CE 20.2461.240 705.398 50 DP 64.2 sp|O14791|APOL1_WT.SETAEELK.+2y6.heavyprimary CE 20.2 461.240 217.082  5 DP 64.2sp|O14791|APOL1_WT.SETAEELK.+2b2.heavy secondary CE 20.2 473.248 846.42  5 DP 65.6 sp|O14791|APOL1_WT.VAQELEEK.+2y7.light secondary CE 21.1473.248 775.383 50 DP 65.6 sp|O14791|APOL1_WT.VAQELEEK.+2y6.lightprimary CE 21.1 480.764 861.452  5 DP 65.6sp|O14791|APOL1_WT.VAQELEEK.+2y7.heavy secondary CE 21.1 480.764 790.41550 DP 65.6 sp|O14791|APOL1_WT.VAQELEEK.+2y6.heavy primary CE 21.1553.304 765.389 10 DP 71.5 sp|O14791|APOL1_WT.LNILNNNYK.+2y6.lightprimary CE 28.6 553.304 652.305 30 DP 71.5sp|O14791|APOL1_WT.LNILNNNYK.+2y5.light secondary CE 24.6 560.819773.403 10 DP 71.5 sp|O14791|APOL1_WT.LNILNNNYK.+2y6.heavy primary CE28.6 560.819 660.319 30 DP 71.5 sp|O14791|APOL1_WT.LNILNNNYK.+2y5.heavysecondary CE 24.6 562.282 896.429 10 DP 72.1sp|O14791|APOL1_G1.LNMLNNNYK.+2y7.light secondary CE 25.0 562.282652.305 30 DP 72.1 sp|O14791|APOL1_G1.LNMLNNNYK.+2y5.light primary CE25.0 569.798 904.444 10 DP 72.1 sp|O14791|APOL1_G1.LNMLNNNYK.+2y7.heavysecondary CE 25.0 569.798 660.319 30 DP 72.1sp|O14791|APOL1_G1.LNMLNNNYK.+2y5.heavy primary CE 25.0 414.751 375.19910 DP 61.4 sp|O14791|APOL1_G2.LNILNNK.+2y3.light secondary CE 18.5414.751 601.367 30 DP 61.4 sp|O14791|APOL1_G2.LNILNNK.+2y5.light primaryCE 22.5 422.266 723.424 10 DP 61.4 sp|O14791|APOL1_G2.LNILNNK.+2y6.heavysecondary CE 18.5 422.266 609.381 30 DP 61.4sp|O14791|APOL1_G2.LNILNNK.+2y5.heavy Primary CE 22.5 422.266 383.213 10DP 61.4 sp|O14791|APOL1_G2.LNILNNK.+2y3.heavy N/A CE 18.5

Sanger Sequencing

Sanger sequencing of genomic DNA employed the following primers.

PCR Amplification ApoL1 F2 (SEQ ID NO. 9)5′-CGACCTGGTCATCAAAAGCCTTGAC-3′ ApoL1 R2 (SEQ ID NO. 10)5′-GGAGGCAGAGCTTGCAGTGAGCTG-3′ Sequencing Reaction ApoL1 F1(SEQ ID NO. 11) 5′-AGACGAGCCAGAGCCAATC-3′ ApoL1 R1 (SEQ ID NO. 12)5′-CTGCCAGGCATATCTCTCCT-3′

Genomic DNA was PCR amplified using a 66° C. annealing temperature for30 cycles. The amplified product was isolated by agarose gelelectrophoresis and the DNA treated with Shrimp AlkalinePhosphatase/Exonuclease I for sequencing.

Data Analysis

Following integration of all chromatographic peaks (e.g., withinAnalyst,) the raw analytical responses (peak areas) were processed asfollows for each specimen:

1. Divide the response of each surrogate peptide's primary transition bythe response of the matching labeled internal standard peptide's primarytransition—this is the primary Analyte: Internal Standard peak arearatio (primary PAR).2. Divide the response of each surrogate peptide's secondary transitionby the response of the matching labeled internal standard peptide'ssecondary transition—this is the secondary Analyte: Internal Standardpeak area ratio (secondary PAR).3. Compare the primary PAR and secondary PAR for each surrogate peptideto the values within the PAR Threshold Table to determine the PARclassifications.

TABLE 4 PAR Threshold Table Qualifying Peptides Variant SpecificPeptides PAR Classification SET VAQ LNI_WT LNM_G1 LNI_G2 PrimaryDetected ≥0.021  ≥0.080  ≥0.043 ≥0.031 ≥0.046 PAR Indeterminate — —0.017-0.043 0.012-0.031 0.018-0.046 Undetected <0.021 <0.080 ≤0.017≤0.012 ≤0.018 Secondary Detected ≥0.021  ≥0.080  ≥0.043 ≥0.066 ≥0.073PAR Indeterminate — — 0.017-0.043 0.038-0.066 0.028-0.073 Undetected<0.021 <0.080 ≤0.017 ≤0.038 ≤0.028

Based on the pattern of surrogate peptides observed in a specimen(Positive=1, Negative=0), the genotype of the specimen may be definedusing the Pattern Table (Table 5). IF the pattern of surrogate peptidesdetected and undetected in a specimen is not found within the PatternTable, then the genotype is deemed inconclusive and the extractedspecimen may be re-injected and/or the original specimen may bere-extracted. Results were compared to results by Sanger sequencing.

TABLE 5 Pattern Table Qualifying Peptides Variant-Specific PeptidesGenotype SET VAQ LNI_WT LNM_G1 LNI_G2 WT/WT 1 1 1 0 0 WT/G1 1 1 1 1 0WT/G2 1 1 1 0 1 G1/G1 1 1 0 1 0 G2/G2 1 1 0 0 1 G1/G2 1 1 0 1 1 Positive1 1 1 1 1 Control Negative 0 0 0 0 0 Control

Example 2—Embodiments

The disclosure may be better understood by referencing the followingnon-limiting embodiments.

A1. A method for determining a genotype of a gene of interest in asubject, the method comprising:

providing a body fluid from the subject, the bodily fluid containing aprotein derived from the gene of interest;

depositing the body fluid on a solid substrate, wherein the fluid isallowed to dry to produce a dry specimen;

digesting the dry specimen to generate at least one allele specificsurrogate peptide for the protein;

using mass spectrometry to detect the at least one allele specificsurrogate peptide present in the digested sample; and

determining the genotype of the subject based on the presence or absenceor amount of the at least one allele specific surrogate peptide.

A2. The method of any of the previous or subsequent embodiments, whereinthe step of digesting is performed with the protease trypsin.A3. The method of any of the previous or subsequent embodiments, whereinthe dry specimen containing the protein derived from the gene ofinterest is denatured prior to digestion.A4. The method of any of the previous or subsequent embodiments, whereinthe at least one allele specific surrogate peptide is analyzed by liquidchromatography tandem mass spectrometry (LC-MS/MS).A5. The method of any of the previous or subsequent embodiments, furthercomprising measuring the amount of at least one common surrogate peptidethat is common to each genotype of the gene of interest.A6. The method of any of the previous or subsequent embodiments, whereinthe presence or absence of the at least one allele-specific surrogatepeptide is determined by comparing a measured response for at least oneallele-specific surrogate peptide to a measured response for at leastone common surrogate peptide.A7. The method of any of the previous or subsequent embodiments, whereinthe presence or absence of the at least one allele specific surrogatepeptide is determined by comparing a measured response for the at leastone allele specific surrogate peptide to a measured response for astable isotope-labeled analogue of the at least one allele specificsurrogate peptide.A8. The method of any of the previous or subsequent embodiments, whereinthe presence or absence of the at least one common surrogate peptide isdetermined by comparing a measured response for the at least one commonsurrogate peptide to a measured response for a stable isotope-labeledanalogue of the at least one common surrogate peptide.A9. The method of any of the previous or subsequent embodiments, whereinthe stable isotope-labeled analogue of the at least one allele specificsurrogate peptide is added as an internal standard.A10. The method of any of the previous or subsequent embodiments,wherein the stable isotope-labeled analogue of the at least one commonsurrogate peptide is added as an internal standard.A11. The method of any of the previous or subsequent embodiments,wherein the measured response of the allele specific surrogate peptideis normalized to the measured response of the stable isotope-labeledanalogue of the at least one allele specific surrogate peptide.A12. The method of any of the previous or subsequent embodiments,wherein the measured response of the common surrogate peptide arenormalized to the measured response of the stable isotope-labeledanalogue of the at least one common surrogate peptide.A13. The method of any of the previous or subsequent embodiments,wherein the internal standard for the allele specific surrogate peptideis added prior to the step of digestion.A14. The method of any of the previous or subsequent embodiments,wherein the internal standard for the common surrogate peptide is addedprior to the step of digestion.A15. The method of any of the previous or subsequent embodiments,wherein the measured response is the peak area ratio for a MS/MStransition characteristic of at least one fragment ion generated fromthe allele specific surrogate peptide.A16. The method of any of the previous or subsequent embodiments,wherein the measured response is the peak area ratio for a MS/MStransition characteristic of at least one fragment ion generated fromthe common surrogate peptide.A17. The method of any of the previous or subsequent embodiments,wherein the protein is ApoL1.A18. The method of any of the previous or subsequent embodiments,wherein the allele specific surrogate peptide has the amino acidsequence LNILNNNYK (SEQ ID NO. 4) derived from the wild-type allele (SEQID NO. 1), or has the amino acid sequence LNMLNNNYK (SEQ ID NO. 5)derived from the G1 allele (SEQ ID NO. 2), or has the amino acidsequence LNILNNK (SEQ ID NO. 6) derived from the G2 allele (SEQ ID NO.3).A19. The method of any of the previous or subsequent embodiments,further comprising determining the amount of a common surrogate peptidehaving the amino acid sequence of SETAEELK (SEQ ID NO. 7) or VAQELEEK(SEQ ID NO. 8) wherein the common surrogate peptide is present in eachof the wild-type, G1 or G2 alleles.A20. The method of any of the previous or subsequent embodiments,wherein the mass spectrometry measures at least one of the transitionsin Table 3.A21. The method of any of the previous or subsequent embodiments,wherein the presence or absence of the at least one allele specificsurrogate peptide is determined by comparing a measured response for theat least one allele specific surrogate peptide to a measured responsefor a stable isotope-labeled analogue listed in Table 2 of the at leastone allele specific surrogate peptide.A22. The method of any of the previous or subsequent embodiments,wherein the liquid chromatography comprises high performance liquidchromatography (HPLC).A23. The method of any of the previous or subsequent embodiments,wherein the dried specimen is dried plasma from separated whole blood.A24. The method of any of the previous or subsequent embodiments,wherein the dried specimen is dried red blood cells from separated wholeblood.A25. The method of any of the previous or subsequent embodiments,wherein the dried specimen is at least one of dried blood, dried urineor dried saliva.B1. A system for determining the genotype of a gene of interest in asubject, the system comprising:

a device for providing and drying a body fluid comprising a proteinderived from the gene of interest;

a station for subjecting the dry body fluid to digestion to generate atleast one allele specific surrogate peptide and optionally, at least onecommon surrogate peptide for the protein;

optionally, a station for chromatographic purification of the at leastone allele specific surrogate peptide and the optional at least onecommon surrogate peptide; and

a station for analyzing the at least one allele specific surrogatepeptide by mass spectrometry to determine the presence or amount of theat least one allele specific surrogate peptide in the biological sample.

B2. The system of any of the previous or subsequent embodiments, whereinthe device for providing a biological sample comprises a device toimmobilize and separate red blood cells from plasma on a substrate.B3. The system of any of the previous or subsequent embodiments, furthercomprising a station for adding a stable isotope labeled internalstandard for the at least one allele specific surrogate peptide andoptionally, at least one common surrogate peptide for the proteinB4. The system of any of the previous or subsequent embodiments, whereinthe station for mass spectrometry comprises a tandem mass spectrometer.B5. The system of any of the previous or subsequent embodiments, whereinthe station for chromatography comprises high performance liquidchromatography (HPLC)B6. The system of any of the previous or subsequent embodiments, whereinat least one of the stations is controlled by a computer.B7. The system of any of the previous or subsequent embodiments, whereinthe protein is ApoL1.

All documents referred to in this specification are herein incorporatedby reference. Various modifications and variations to the describedembodiments of the inventions will be apparent to those skilled in theart without departing from the scope and spirit of the invention.Although the invention has been described in connection with specificpreferred embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiments.Indeed, various modifications of the described modes of carrying out theinvention which are obvious to those skilled in the art are intended tobe covered by the present invention.

REFERENCE TO A SEQUENCE LISTING SUBMITTED AS A TEXT FILE VIA EFS-WEB

The official copy of the sequence listing is submitted electronicallyvia EFS-Web as an ASCII formatted sequence listing with a file named1139048_ST25.txt, created on Jun. 23, 2019, and having a size of 14.0kilobytes. The sequence listing contained in this ASCII formatteddocument is part of the specification and is herein incorporated byreference in its entirety.

That which is claimed:
 1. A method for determining a genotype of a geneof interest in a subject, the method comprising: providing a driedplasma sample containing a protein derived from the gene of interest,wherein the dried plasma sample is deposited on a solid substrate andcomprises dried plasma separated from red blood cells; digesting thedried plasma sample to generate at least one allele specific surrogatepeptide for the protein, thereby generating a digested sample; usingmass spectrometry, detecting the at least one allele specific surrogatepeptide in the digested sample; and determining the genotype of thesubject based on results of the detecting the at least one allelespecific surrogate peptide.
 2. The method of claim 1, wherein thedigesting is performed using a protease.
 3. The method of claim 1,wherein the dried plasma sample containing the protein derived from thegene of interest is subjected to denaturation prior to the digesting. 4.The method of claim 1, wherein the mass spectrometry is tandem massspectrometry.
 5. The method of claim 4, wherein the tandem massspectrometry is liquid chromatography tandem mass spectrometry(LC-MS/MS).
 6. The method of claim 1, further comprising detecting inthe digested sample, using the mass spectrometry, at least one commonsurrogate peptide that is common to each genotype of the gene ofinterest.
 7. The method of claim 6, wherein the detecting the at leastone common surrogate peptide comprises acquiring a measured response forthe at least one common surrogate peptide.
 8. The method of claim 7,wherein the measured response for the at least one common surrogatepeptide is a peak area ratio for a MS/MS transition characteristic of atleast one fragment ion generated from the common surrogate peptide. 9.The method of claim 7, wherein presence or absence of the at least onecommon surrogate peptide is determined by comparing the measuredresponse for the at least one common surrogate peptide to a measuredresponse for a stable isotope-labeled analogue of the at least onecommon surrogate peptide.
 10. The method of claim 7, wherein thedetecting the at least one allele specific surrogate peptide in thedigested sample comprises acquiring a measured response for the at leastone allele specific surrogate peptide, and wherein presence or absenceof the at least one allele specific surrogate peptide in the digestedsample is determined by comparing the measured response for the at leastone allele specific surrogate peptide to the measured response for theat least one common surrogate peptide.
 11. The method of claim 7,wherein presence or absence of the at least one common surrogate peptidein the digested sample is determined by comparing the measured responsefor the at least one common surrogate peptide to a measured response fora stable isotope-labeled analogue of the at least one common surrogatepeptide.
 12. The method of claim 1, wherein the detecting the at leastone allele specific surrogate peptide comprises acquiring a measuredresponse for the at least one allele specific surrogate peptide.
 13. Themethod of claim 12, wherein the measured response for the at least oneallele specific surrogate peptide is a peak area ratio for a MS/MStransition characteristic of at least one fragment ion generated fromthe allele specific surrogate peptide.
 14. The method of claim 12,wherein presence or absence of the at least one allele specificsurrogate peptide in digested sample is determined by comparing themeasured response for the at least one allele specific surrogate peptideto the measured response for the at least one common surrogate peptide.15. The method of claim 12, wherein presence or absence of the at leastone allele specific surrogate peptide in the dried plasma sample isdetermined by comparing the measured response for the at least oneallele specific surrogate peptide to a measured response for a stableisotope-labeled analogue of the at least one allele specific surrogatepeptide.
 16. The method of claim 1, wherein the detecting the at leastone allele specific surrogate peptide comprises detecting an amount ofthe at least one allele specific surrogate peptide.
 17. The method ofclaim 1, wherein the dried plasma is produced by separating plasma fromthe red blood cells on the solid substrate.
 18. A method for determininga genotype of a subject, the method comprising: providing a dried plasmasample, wherein the dried plasma sample is deposited on a solidsubstrate and comprises dried plasma obtained from the subject andseparated from red blood cells; digesting the dried plasma sample togenerate a plurality of allele specific surrogate peptides for proteinvariants contained in the dried plasma, thereby generating a digestedsample; using mass spectrometry, detecting the plurality of the allelespecific surrogate peptides in the digested sample to generate aproteomic profile of the allele specific surrogate peptides; anddetermining the genotype of the subject based on the proteomic profile.19. A system for determining genotype of a gene of interest in asubject, the system comprising: a station for subjecting a dried plasmasample to digestion to generate at least one allele specific surrogatepeptide for a protein derived from the gene of interest, wherein thedried plasma sample is deposited on a solid substrate and comprisesdried plasma separated from red blood cells; optionally, a station forchromatographic purification of the at least one allele specificsurrogate peptide; a station for analyzing the at least one allelespecific surrogate peptide by mass spectrometry to detect the at leastone allele specific surrogate peptide; and, a station for determiningthe genotype of the gene of interest based on results of the of the atleast one allele specific surrogate peptide.
 20. The system of claim 20,wherein the station for analyzing the at least one allele specificsurrogate peptide by mass spectrometry comprises a tandem massspectrometer.